The psychology of making money.
Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.
Those three are the id, the super-ego and the ego. The id lives deep within the subconscious from birth. It's the source of our sudden and most instinctive urges. When you find yourself doing what you know you shouldn't, blame your id.
The super-ego is formed later in life to balance the id. It forms and breaks off from the id through our upbringing, parental guidance and our sense of community and beliefs. People who are careful with their money can thank their super-ego.
The ego is the driver making the decisions. It decides between the devil (the id) and the angel (the super-ego) on either shoulder (yes, all those cartoons you've ever seen are partly true). We have voices in our mind, and it's up to the ego to decide which one to fulfill. Its goal is to satisfy the id in some way while also attending to the super-ego.